A known method of microscopically examining a spatial fine structure is applied in the production of electronic semiconductor devices for checking conductor fine structures or isolator fine structures. Here, the respective fine structure is metallised with aluminium, for example, and then electron-microscopically examined. Electron-microscopy is used to spatially resolve the fine structure as far as possible. However, the efforts to be taken for electron-microscopic examination are high. The object having the fine structure has to be locked in a high vacuum apparatus already for metallization with the oxidation-sensitive aluminium; electron-microscopic examination can always only be executed in high vacuum. This means that no volatile matters may be emitted by the fine structure, which could deteriorate the high vacuum and/or damage the high vacuum apparatus. The efforts for installation and use of an electron-microscope itself are also quite high. Additionally, it has to be regarded as a disadvantage that the fine structures which have been examined electron-microscopically have to be discarded because of their irreversible coating with aluminium. I.e. the known method cannot be applied to check a fine structure which is as such also present in a final product. Instead, it is only possible to take lost samples.
At present, very few alternatives to electron-microscopic examination are available in the lithographic production of fine structures, if a resolution in the range of less than 150 nm is to be obtained. Present printed circuit board tracks in microelectronics already comprise widths down to 90 nm with even lower track distances. The recognisable alternatives to electron-microscopy are methods in which the fine structure to be examined is scanned with a probe. Atomic Force Microscopy (AFM) and Scanning Near Field Optical Microscopy (SNOM) belong to these methods, which on the one hand need no high vacuum, but which are on the other hand dependent on an exact and thus laborious adjustment of the clean fine structure with regard to the sensible arrangement for moving the respective probe, and which are thus extremely slow as compared to the size of the fine structure to be examined. The efforts to be taken for microscopic examination according to the known methods thus seem to be significantly reducible only in that very few samples of the fine structures generated lithographically are examined. This, however, essentially increases the danger of faultily produced electric devices.
A method of fluorescence-microscopically examining a sample is for example known from U.S. Pat. No. 7,253,893. A fluorescence dye, by which structures of interest within a sample have been dyed in a previous step, is at first transferred into an excited energetic state by means of an exciting optical signal. In this optical excitation the usual limit of λ/(2n sinα) for spatial resolution in optical methods applies, λ being the wavelength of the light used, n being the refraction index of the sample, and α being the half aperture angle of the objective used. To get below this limit, the optically excited state of the fluorescence dye is de-excited again with a de-exciting optical signal outside a desired measurement point in which the de-exciting optical signal has a zero point; i.e. the fluorescence dye in the sample is forced to stimulated emission everywhere outside the measurement point by means of the optical signal. The dimensions of the resulting still fluorescent measurement point, i.e. the spatial resolution of the remaining fluorescence can be lowered clearly below the usual optical resolution limit in that the de-exciting optical signal is applied to the sample outside the desired measuring point at such an intensity, that a saturation in de-excitation by stimulated emission is achieved. Thus, the fluorescence dye in the sample only remains in the excited state in a strongly delimited area about the zero point of the intensity distribution of the de-exciting optical signal and can only fluoresce in this area.
According to Hell, Nature Biotech., 21, 1347-1355. the size of the fluorescent measuring point Δx and thus the resolution follows Δx ≈λ/(2n sin α√(I/Is)), λ being the wavelength of the de-exciting optical signal, n being the refraction index of the sample, αbeing the half aperture angle of the objective used, I being the applied intensity of the de-exciting optical signal, and Is being the saturation intensity. The saturation intensity Is is the characteristic intensity at which the fluorescence dye in the sample can be de-excited by application of the de-excitation optical signal by 50% from a statistics point of view.